1. Field of the Invention
The present invention relates to a direct conversion type radiation detector for use in a medical field, an industrial field, and a nuclear energy field. More particularly, the present invention relates to techniques for improving an environment resistance of a radiation sensitive type semiconductor film in the radiation detector and for suppressing a creeping discharge due to a bias voltage applied to the radiation sensitive type semiconductor film.
2. Description of the Related Art
Detectors for detecting radiation, such as X-rays, include indirect conversion type detectors, and direct conversion type detectors. The indirect conversion type detectors are adapted to first convert radiation into light and then perform photoelectric conversion of the converted light into electric signals. The direct conversion type detectors are adapted to convert incident radiation directly into electrical signals, such as a radiation sensitive type semiconductor film.
In the latter direct conversion type detector, a predetermined bias voltage is applied onto a voltage application electrode formed on a front surface of a radiation sensitive type semiconductor film. A carrier collection electrode is formed on a back surface of the semiconductor film and collects carriers, which are generated by radiation irradiation and extracts the collected carriers as a radiation detection signal thereby to perform detection of radiation.
Further, when a thick film of an amorphous semiconductor, such as amorphous selenium, is used as a radiation sensitive type semiconductor film, a thick and large film of an amorphous semiconductor can easily be formed by a vacuum evaporation method. Thus, the amorphous semiconductor is suitable for constructing a two-dimensional array type radiation detector, which requires a large area thick film, among the related art conversion type radiation detectors
As illustrated in FIG. 8, a related art two-dimensional array type radiation detector comprises an insulating substrate 86, an amorphous semiconductor thick film 81 and a voltage application electrode 82. The insulating substrate 86 has plural charge storage capacitors Ca and plural charge-reading switching elements 88, which are formed in a crosswise or two-dimensional matrix-like arrangement thereon. The charge-reading switching element 88 is constituted by thin film transistors and is normally put in an OFF-state. The amorphous semiconductor thick film 81 is electrically connected to the plural charge storage capacitors Ca and formed on the insulating substrate 86 through plural carrier collection electrodes 87. In amorphous semiconductor thick film 81, Charge transfer media (that is, carriers) are generated by incidence of radiation. The voltage application electrode 82 is formed on a surface of the amorphous semiconductor thick film 81. Incidentally, one charge storage capacitor Ca and one charge reading switching element 88 are provided correspondingly to each of the carrier collection electrodes 87. Each set of the charge storage capacitor Ca, the charge reading switching element 88, and the carrier collection electrodes 87 constitutes a detecting element DU serving as a radiation detection unit.
Incidentally, when radiation is irradiated during a state in which a bias voltage is applied to the voltage application electrode 82, the charge transfer media (that is, carriers) generated by incidence of radiation are respectively moved by the bias voltage to the voltage application electrode 82 and the carrier collection electrode 87. Electric charges are stored in the charge storage capacitor Ca according to the number of the generated carriers. The stored charges are read as radiation detection signals by putting the switching element 88 into an ON-state.
When the radiation detector of the two-dimensional array configuration shown in FIG. 8 is used for detecting, for example, an X-ray fluoroscopic image obtained by an X-ray fluoroscopic imaging system, the X-ray fluoroscopic image is obtained according to the radiation detection signal outputted from the radiation detector.
However, in the case of the related art radiation detector, the thermal expansion coefficient of the amorphous semiconductor thick film 81 is large. Thus, there is a danger that thermal warpage occurs owing to the difference in the thermal expansion coefficient between the amorphous semiconductor thick film 81 and the substrate 86. When such warpage occurs, the amorphous semiconductor thick film 81 may crack. In such a case, such cracks may result in image defects. Further, discharge breakdown may occur at a crack portion, and the detector may be brought into an inoperable state.
Furthermore, in the case of the related art radiation detector, as illustrated in FIG. 8, read lines 810, gate lines 811, and ground lines 812 have parts 810a, 811a, and 812a exposed on the insulating substrate 86. Therefore, there is a danger that a creeping discharge is caused by an occurrence of dielectric breakdown in a portion from an end edge 82a of the voltage application electrode 82 along the surface of an end edge 81a of an amorphous semiconductor thick film 81 to the parts 810a, 811a, and 812a. Further, for example, in the case of an X-ray fluoroscopic image, an occurrence of a creeping discharge causes noises of the radiation detection signal. This results in degradation in the picture quality of the image. Creeping discharge can be suppressed by setting the bias voltage at a low value as a countermeasure there against. However, in such a case, the amorphous semiconductor is inferior in carrier transit characteristics to a monocrystalline semiconductor. Thus, the related art radiation detector has encountered a drawback in that the detector cannot obtain sufficient detection sensitivity.
Incidentally, to deal with the problem of the environment resistance, such as that of warpage due to change in temperature, there has been proposed a radiation detector A having an insulating plate member 95 as shown in FIG. 9. The insulating plate member 95 is formed on the surfaces of the top layer of the amorphous semiconductor thick film 91 and the voltage application electrode 92 formed on the insulating substrate 96. The insulating plate member 95 has thermal expansion coefficient which is comparable to that of the insulating substrate 96. The insulating plate member 95 is fixed by a high-withstand-voltage hardening synthetic resin 94 in such a manner as to cover the entire surfaces of the top layer.
However, in such a radiation detector A, a result of an experiment is obtained, which reveals that the surface of the amorphous semiconductor thick film 91 is deteriorated by a solvent ingredient of the high-withstand-voltage hardening synthetic resin 94, so that a creeping discharge occurs and the withstand voltage lowers (see a result of an experiment corresponding to a first comparative detector, which is shown in FIG. 7).
Moreover, to prevent an occurrence of a creeping discharge, there has been proposed a radiation detector B having a carrier-selective high-resistance film 103 made of a material, such as Sb2S3, which is formed between an amorphous semiconductor thick film 101 and a voltage application electrode 102, as shown in FIG. 10 (in Japanese Patent Application No. 11-240026). The carrier-selective high-resistance film 103 is formed in such a way as to entirely cover the surface of the amorphous semiconductor thick film 101, which is liable to deteriorate.
However, such a radiation detector B has a drawback in that the carrier-selective high-resistance film 103 made of a material, such as Sb2S3, is inferior in tensile strength and thus cannot withstand warpage of the amorphous semiconductor thick film 101, which is caused owing to change in temperature, and that cracks are apt to occur. Further, it is difficult for the carrier-selective high-resistance film 103 to have a thickness which is sufficient for preventing an occurrence of a creeping discharge. Incidentally, each of a switching element 108 and a carrier collection electrode 107 shown in FIG. 10 has a structure similar to that of a corresponding one of the switching element 88 and the carrier collection electrode 87 shown in FIG. 8.
Furthermore, to prevent an occurrence of a creeping discharge, there has been proposed a radiation detector C having a discharge preventing film 110 made of high-withstand-voltage insulating materials, such as silicon resin and epoxy resin, as shown in FIG. 11 (in Japanese Patent Application No. 2000-191167) The discharge preventing film 110 is formed on the surfaces of the top layer of an amorphous semiconductor thick film 111 and a voltage application electrode 112.
However, in such a radiation detector C, the discharge preventing film 110 made of high-withstand-voltage insulating materials, such as a silicon resin and an epoxy resin, differs in thermal expansion coefficient from and relatively inferior in surface strength to an insulating substrate 116. Thus, similarly, warpage due to change in temperature occurs. Consequently, cracks occur in the resin itself and the interlayer 113 between the amorphous semiconductor thick film 111 and the voltage application electrode 112, so that the detector has an insufficient creeping discharge withstand voltage (see a result of an experiment corresponding to a second comparative detector, which is shown in FIG. 7).
Incidentally, each of a switching element 118 and a carrier collection electrode 117 shown in FIG. 11 has a structure similar to that of a corresponding one of the switching element 88 and the carrier collection electrode 87 shown in FIG. 8.
In view of the aforementioned circumstances, an object of the invention is to provide a radiation detector that is superior in an environment resistance, such as temperature resistance, and is enabled to prevent an occurrence of a creeping discharge due to a bias voltage applied to a radiation sensitive type semiconductor film and to obtain sufficient detection sensitivity.
To achieve the foregoing object, according to the invention, there is provided a radiation detector (hereunder referred to as a first radiation detector of the invention), which comprises an insulating substrate having a charge storage capacitor and a charge reading switching element which are formed thereon; a carrier collection electrode formed on the insulating substrate and electrically connected to the charge storage capacitor; a radiation sensitive type amorphous semiconductor thick film formed on the carrier collection electrode and adapted to generate charge transfer media in response to an incidence of a radiation; a voltage application electrode formed on the amorphous semiconductor thick film; a solvent-resistant and carrier-selective high-resistance film formed between the amorphous semiconductor thick film and the voltage application electrode in such a manner as to cover the entire surface of the amorphous semiconductor thick film; and an insulating auxiliary plate member having a thermal expansion coefficient, which is comparable to that of the insulating substrate, and being formed on the voltage application electrode and fixed thereto by a high-withstand-voltage hardening synthetic resin in such a way as to prevent warpage of the amorphous semiconductor thick film from occurring owing to change in temperature. In the radiation detector, electric charges are stored in the charge storage capacitor according to the number of the charge transfer media generated in the amorphous semiconductor thick film by the incidence of radiation and the electric charges stored in the charge storage capacitor is read as a radiation detection signal through the switching element.
According to an embodiment (hereunder referred to as a second radiation detector of the invention) of the first radiation detector of the invention, the radiation detector has a two-dimensional array structure in which a plurality of carrier collection electrodes are formed in such away as to be arranged in a two-dimensional matrix-like manner, and one of charge storage capacitors and one of charge reading switching elements are provided correspondingly to each of the carrier collection electrodes.
According to an embodiment (hereunder referred to as a third radiation detector of the invention) of the first radiation detector of the invention, the solvent-resistant and carrier-selective high-resistance film is a Sb2S3 film having a thickness ranging from 0.05 xcexcm to 10 xcexcm.
According to an embodiment (hereunder referred to as a fourth radiation detector of the invention) of the first radiation detector of the invention, the high-withstand-voltage hardening synthetic resin is an epoxy resin.
According to an embodiment (hereunder referred to as a fifth radiation detector of the invention) of the first radiation detector of the invention, the auxiliary plate member includes a material containing elements, each of which has an atomic number of 15 or more and the total content of which is 1 atomic % or less.
According to an embodiment (hereunder referred to as a sixth radiation detector of the invention) of the first radiation detector of the invention, the auxiliary plate member has a structure in which a thickness of a part thereof located in a radiation sensitive region is less than that of a part thereof located in an insensitive region.
According to an embodiment (hereunder referred to as a seventh radiation detector of the invention) of the first radiation detector of the invention, the auxiliary plate member has a structure in which only a part thereof located in a radiation sensitive region is opened.
An embodiment (hereunder referred to as an eighth radiation detector of the invention) of the first or second radiation detector of the invention further comprises a spacer for adjusting a gap between the auxiliary plate member and the insulating substrate.
According to an embodiment (hereunder referred to as a ninth radiation detector of the invention) of the eighth radiation detector of the invention, the gap between the insulating substrate and the auxiliary plate member ranges from 2 mm to 4 mm.
Next, an operation of each of the radiation detectors of the invention is described hereinbelow.
In the case that the radiation detection is performed by the radiation detector of the invention, radiation to be detected is incident on the detector while the bias voltage is applied to the voltage application electrode formed on the front surface side of he radiation sensitive type amorphous semiconductor thick film. The charge transfer media (that is, carriers) generated in the amorphous semiconductor thick film by the incidence of the radiation are moved by the bias voltage to the voltage application electrode and the carrier collection electrode. Then, electric charge is stored in the charge storage capacitor electrically connected to the carrier collection electrode according to the generated carriers. Moreover, as the transition of the state of the charge reading switching element to an ON-state is performed, the stored charge is read as the radiation detection signal through the switching element.
Furthermore, in the case of the radiation detector of the invention, the solvent-resistant and carrier-selective high-resistance film is formed between the amorphous semiconductor thick film and the voltage application electrode in such a manner as to cover the entire surface of the amorphous semiconductor thick film. Thus, the phenomena, in which the surface of the amorphous semiconductor thick film is deteriorated by the solvent ingredient of the high-withstand-voltage hardening synthetic resin, and in which a creeping discharge occurs and the withstand voltage lowers, do not occur. Furthermore, increase in dark current is suppressed.
Moreover, the insulating auxiliary plate member is formed on and fixed to the top layer surface, on which the amorphous semiconductor thick film, the solvent-resistant and carrier-selective high-resistance film, and the voltage application electrode are formed, and fixed thereto by the high-withstand-voltage hardening synthetic resin in such a way as to cover the top layer surface. Thus, the amorphous semiconductor thick film, the solvent-resistant and carrier-selective high-resistance film, and the voltage application electrode, which are relatively inferior in tensile strength, are sandwiched by the insulating plate members that are nearly equal in thermal expansion coefficient to each other. Consequently, the frequency of occurrences of warpage of the amorphous semiconductor thick film owing to change in temperature and that of occurrences of cracks therein are sharply decreased.
Further, a path on which dielectric breakdown other than the breakdown of the amorphous semiconductor thick film itself is assumed to occur, is that extending through the high-withstand-voltage hardening synthetic resin film along the interface between the high-withstand-voltage hardening synthetic resin film and the auxiliary plate member to the part in which the read lines, the gate lines and the ground lines are exposed on the insulating substrate, and that extending to apart in which the recombination of the charge carrier caused by the voltage application electrode and electric charge produced on the auxiliary plate member. Thus, an occurrence of dielectric breakdown is prevented by forming the high-withstand-voltage hardening synthetic resin in such a way as to have a thickness, which is sufficiently thick to the extent that no dielectric breakdown is caused by the bias voltage.
Moreover, the insulating auxiliary plate member is formed on and fixed to the surface of the top layer, in which the amorphous semiconductor thick film, the solvent-resistant and carrier-selective high-resistance film, and the voltage application electrode are formed, and fixed thereto by the high-withstand-voltage hardening synthetic resin in such a way as to cover the top layer surface. This structure serves as a protective film for the amorphous semiconductor thick film that is relatively inferior in the environment resistance.
Further, in the case of the second radiation detector of the invention, a two-dimensional array structure, in which the radiation detection units are arranged in a matrix-like form, is formed by providing a charge storage capacitor and a charge reading switching element correspondingly to each of the plural carrier collection electrodes arranged in a two-dimensional matrix-like form. Thus, local radiation detection is performed in each of the radiation detection units.
In the case of the third radiation detector of the invention, the solvent-resistant and carrier-selective high-resistance film is a Sb2S3 film having a thickness ranging from 0.05 xcexcm to 10 xcexcm. The third radiation detector of the invention performs both of an operation of preventing the surface of the amorphous semiconductor thick film from being deteriorated by the solvent ingredient of the high-withstand-voltage hardening synthetic resin, and of preventing an occurrence of the creeping discharge and the lowering of the withstand voltage, and another operation of preventing increase in dark current.
In the case of the fourth radiation detector of the invention, the high-withstand-voltage hardening synthetic resin is an epoxy resin. Thus, the fourth radiation detector of the invention features that this resin is superior in strength thereof after the hardening, whereas the reactiveness of the solvent ingredient of the resin to the amorphous semiconductor film is low.
In the case of the fifth radiation detector of the invention, the auxiliary plate member is constituted by a material containing elements, each of which has an atomic number of 15 or more and the total content of which is 1 atomic % or less. Thus, the attenuation of the incident radiation can be suppressed.
In the case of the sixth radiation detector of the invention, the auxiliary plate member has a structure in which a thickness of a part thereof located in a radiation sensitive region is less than that of apart thereof located in an insensitive region. Thus, the attenuation of the incident radiation can be suppressed still more by simultaneously maintaining the strength against the warpage of the amorphous semiconductor thick film, which is caused owing to change in temperature, and against cracks occurring therein.
In the case of the seventh radiation detector of the invention, only a part thereof located in a radiation sensitive region is opened. Thus, the attenuation of the incident radiation can be minimized by simultaneously maintaining the strength against the warpage of the amorphous semiconductor thick film, which is caused owing to change in temperature, and against cracks occurring therein.
In the case of the eighth radiation detector of the invention, the spacer for adjusting the bonding gap between the auxiliary plate member and the insulating substrate is provided at a peripheral portion of the auxiliary plate member. This facilitates the control of the bonding gap. Moreover, variation in sensitivity due to variation in attenuation of the incident radiation can be suppressed.
In the case of the ninth radiation detector of the invention, the bonding gap between the insulating substrate and the auxiliary plate member is set to be more than or equal to 2 mm. Thus, among the paths on which dielectric breakdown other than the breakdown of the amorphous semiconductor thick film itself is assumed to occur, it is possible to lengthen the path extending through the high-withstand-voltage hardening synthetic resin film along the interface between the high-withstand-voltage hardening synthetic resin film and the auxiliary plate member to the part in which the read lines, the gate lines and the ground lines are exposed on the insulating substrate, and to lengthen the distance between the voltage application electrode and the auxiliary plate member so that the amount of electric charge, which the surface of the insulating plate member bears, is reduced Thus, a high withstand voltage, which is equal to or higher than 30 kV, can be obtained. Further, the bonding gap is set to be equal to or less than 4 mm. Consequently, the attenuation of the incident radiation owing to the hardening synthetic resin can be minimized.